1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to an image reading apparatus of an image processing apparatus having, for example, a function of reading document images, such as a scanner, a copier, a facsimile, or a multi-function device having their functions.
2. Description of the Related Art
In an image reading apparatus of an image processing apparatus having, for example, a function of reading document images, such as a scanner, a copier, a facsimile, or a multi-function device having their functions, for example, a solid-state image sensor represented by a CCD (Charge Coupled Device) type image sensor (hereinafter referred to as a CCD sensor) is used as a unit that optically reads document images.
A CCD sensor used by an image reading apparatus generally has a configuration of a so-called line sensor including: a pixel array section in which a large number of pixels each including a photoelectric transducer are linearly arrayed; a CCD register that sequentially horizontally transfers signal charges vertically transferred (shifted) from the individual pixels of the pixel array section; and an output amplifier including a charge-to-voltage conversion section that converts the signal charges sequentially transferred by the CCD register into voltages and outputs the voltages.
To drive the CCD sensor, a main clock (horizontal transfer clock) for driving the CCD register and a last stage pulse (reset pulse, etc.) for driving the output amplifier are used. The main clock is supplied to the CCD register only in a horizontal transfer period in which signal charges are sequentially transferred by the CCD register, and the supply of the main clock to the CCD register is halted during a vertical transfer period in which signal charges are vertically transferred from the individual pixels of the pixel array section to the CCD register. The operations of supply and halt of the main clock for the CCD register are repeated every one-line cycle (one fast-scanning period).
As a result of the supply and halt of the main clock being repeated, a large load current flows through the driver circuit for the main clock in the horizontal scanning period, while few currents flow in the vertical transfer period. As a result, when the vertical transfer period ends and the horizontal transfer starts, a large fluctuation of current consumption occurs in the driver circuit. The fluctuation of current consumption is accompanied by fluctuations of a supply voltage of the driver circuit, which in turn causes fluctuation of voltage amplitudes and the like of the main clock itself.
As a result, a level fluctuation corresponding to fluctuations of a supply voltage of the driver circuit occurs in an output signal of the CCD sensor. Consequently, a black level (reference level of output signal) obtained in a leading portion of one line fluctuates, and the influence of the fluctuation remains in pixel signals of an optical shield pixel portion and pixel signals of an effective pixel portion. As a result, an image signal of normal level cannot be obtained as an output signal of the CCD sensor, with the result that highly accurate reading operations cannot be performed.
Recently, the frequency of a drive signal is set high to increase a reading speed, or the number of pixels of the pixel array section of the CCD sensor is increased to increase read resolution. Thus, as the frequency of the drive signal becomes higher and the number of pixels of the pixel array section of the CCD sensor increases, a load current of the driver circuit for the main clock tends to increase because an input load capacity of the CCD sensor increases. Therefore, level fluctuations of output signals of the CCD sensor become greater which correspond to power supply fluctuations of the driver circuit caused by the repeated supply and halt of the main clock.